Project A - The inactivation of phosphofructokinase in extracts of Tetrahymena pyriformis cells is reversible. Loss of activity does not appear to involve phosphorylation of the enyme. Project B - The rates of loss of specific enzyme activities during starvation of Escherichia coli cells show considerable variation. The relative rates of inactivation of the various enzymes are different depending on whether glucose or ammonia is limiting. There are large differences in the rates of decline of some enzyme activities when different strains of E. coli are starved under similar conditions.